Plastics are versatile materials in large part because they include carbon atoms as their backbone or base. Carbon can form stable bonds with other atoms, such as hydrogen and oxygen, to form a molecule that is monomer. Carbon can also form stable bonds with other carbon atoms while also being bonded to other atoms. In these situations the carbon in two or more monomer molecules may bond with each other to form a molecule that is a polymer. Such polymers can have any number of monomers bonded together. For example, some polymer molecules have three monomers and some polymer molecules have thousands of monomers that together form a molecule having a long chain of monomers. In either case, each of the polymers is a single molecule whose atoms are the same as the others, but whose mass and thus size is very different than the others. Because isolating or forming polymers having the same size is typically difficult and expensive to do, most plastic material includes many molecules/polymers whose atoms are the same but whose sizes are different. And, thus two plastic materials that are the same often do not have the same number and/or distribution of specific same-sized molecules.
This difference in the number and/or distribution of specific same-sized molecules can cause different lots of the same material processed in the same way to result in a product that is not the same. For most end products the difference in the product does not cause enough of a problem to warrant changing the size of the molecules before processing the material. But, for some end products the difference in the product may be significant, and the significance may be beneficial or detrimental depending on the intended use of the end product.